Single
Storm
in Mexico Kills
75%
of Eastern North America?s Migratory Monarchs
January
2002
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- Sierra Chincua Sanctuary: 74% of the butterflies were killed
- El Rosario Sanctuary: 80% of the population was killed
- At the two colonies combined, an estimated 200-272 million butterflies were killed.
Significantly, these two huge colonies are the winter sanctuaries of 2/3 of eastern North America?s migratory butterflies. The other 1/3 of the butterflies are spread among other smaller sites in the vicinity. While scientists have not yet visited these outlying sites, mortality rates are feared to be similar because the sites are small, their forest habitat is less pristine, and because the rain and cold were prolonged in the region.
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Dr. Brower described the catastrophe in an interview with
Journey North: ?The ground in these two colonies was littered with monarchs
that had an eerie flat, pallid appearance that I have never before seen--like
wet leaves. The heavily packed piles of butterflies were up to 13 inches
deep, and even those that were still alive may not have been able to struggle
out. The main survivors were buried alive, covered by dead butterflies
that were insulating them (from the cold).?
In an interview with National Public Radio Brower said, ?The most macabre
aspect of the whole thing was sticking my hands down in and realizing
at the bottom of the pile there were a whole bunch of them still alive.?
Killing Conditions: Wet Followed by Extreme Cold
For
monarchs, the weather pattern that occurred in mid-January was the recipe
for disaster: Heavy rains were followed by clearing skies and plummeting
temperatures. This storm was an historic extreme in that the rains were
heavy during the peak of the dry season in Mexico, and the temperatures
were exceptionally cold. Monarchs are essentially tropical butterflies and
cannot tolerate sub-freezing temperatures for very long. When they are wet,
they die at warmer temperatures than they would if dry. Said Brower, ?Never in the 27 years we?ve been studying in Mexico have we known temperatures to have fallen this low. I think the temperatures dropped below -8C and froze all the wet butterflies...except those in the middle of the clusters...?
It?s important to emphasize that this year?s storm was an extreme, and a high level of mortality may have occurred even with the best forest cover. However, for scientists, conservationists and government officials alike, the historic storm of 2002 will serve as a cautionary reminder: If deforestation continues at its present rate, a single storm even less severe would be capable of decimating the over-wintering population.
?There's always a probability of a severe storm like this,? Brower told NPR, ?but at the hands of humans there's been a 44 percent degradation of the forest over the past 28 years. And the rate of forestry in the area is increasing. This storm shows that we need to really protect the central core area where their survival is really threatened.?
Meteorologist?s Perspective: Satellite Views of the Storm
We wondered whether satellite images were available to document the strength and scope of the storm and cold front. Dr. Dave Dempsey of San Francisco State University responded with these impressive animated video loops showing the jet stream, visible and infrared views. (The sanctuary region is marked with a red "+".) From Dr. Dempsey:
"A cold front swept across central Mexico from the northwest, followed by cold, clear weather. The infrared and visible satellite loops (below left and center) show the broad band of clouds that accompanied the cold front and produced rain and even snow. A cold front is the leading edge of a "tongue" of cold air that protrudes southward from its source farther north. High in the atmosphere, the jet stream blows around the edges of the tongue, forming a dip or "trough" in the jet stream pattern. The jet stream loop (below right) shows a narrow trough dropping unusually far south into central Mexico, indicating the arrival of a tongue of cold air." From here, the most useful addition would be a quantitative estimate of the relative rarity of the January event which requires a familiarity with the climatological data base that I don't have (though I have no doubt that someone out there does)."
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Natural Selection: Meet the Survivors
From
an evolutionary perspective, scientists are interested in this event as
an example of natural selection at work. Just think: The monarchs that migrate
across Eastern North America in the future--perhaps to your own backyard--will
be the offspring of those that survived the storm. Any characteristic that
helped them to survive will be in the gene pool of future monarchs. For
example, might these survivors:
- Be more tolerant to cold than the ones that died?
- Be stronger, fatter, or otherwise more physically fit than those who died?
- Can you think of other possible differences, physical or behavioral, that enabled some to survive?
...or was it just chance!?
Outlook for the Future
Scientists are now left wondering how long it will take for the monarch population to rebound. Mexico's over-wintering sites harbor all of eastern North America's migratory monarch breeding stock. What long-term effect will this storm have on monarch populations, given such a high mortality rate?
First, it?s important to remember that monarchs, as insects, are capable of high reproduction rates. A single female can lay hundreds of eggs. Second, scientists simply don?t know enough about monarch population dynamics to make predictions with confidence. Dr. Orley Taylor, Director of Monarch Watch, wrote an excellent analysis of the storm (see link below) and made this important observation:
"This is the lowest known number of butterflies at these sites over the last 27 years. In retrospect, it appears to be fortunate that the January freeze occurred this year, a year in which the overwintering population was robust (about 100 million butterflies). Even though estimated mortality due to the January freeze is extremely high (>80%) the number of surviving butterflies may be sufficient to recolonize the breeding areas without a long-term depression of the population. Had this storm occurred last season when the overwintering population was at an all time low (28.3 million), it is likely that it would have taken the population many years to return to normal levels of 60-120 million overwintering butterflies.?
Storm in Monarch Sanctuaries: Links to Additional Information
- New York Times: ?Storm in Mexico Devastates Monarch Butterfly Colonies
- All Things Considered: National Public Radio:?Mexico Storm Kills Monarch Butterflies?
Spring Migration Data Takes on New Importance
In the aftermath of this event, scientists will watch with interest as the remaining butterflies move north this spring to breed.
?It will be important to follow the population closely this spring as it moves north," said Dr. Karen Oberhauser, who encourages volunteers to participate in Journey North's migration study and her own study of monarch reproduction, the Monarch Larva Monitoring Project. "...this will give us realistic estimates of the cascading effects of the storm mortality.?
Journey North: Spring Monarch Migration Study
Observers report the first sightings of adult monarchs, the emergence of first milkweed (the host-plant monarchs need to reproduce), and the first evidence of reproduction (first eggs, first larva).
Monarch Larva Monitoring Project
The overarching goal of the project is to better understand how and why monarch populations vary in time and space, with a focus on monarch distribution and abundance during the breeding season in North America. On a weekly basis throughout the breeding season, observers monitor monarch reproduction at local sites.
Estimating Monarch Mortality: The Scientists? Methods
And this exactly what the team did. As these pictures show, Dr. Brower, Dave Kust, his family and visiting friends, collected hundreds and hundreds of butterflies from the sanctuary floor. They selected butterflies by making quadrants measuring .25 meters square, chosen randomly in the colony. They gathered all of the butterflies from inside each measured area. These are called ?measured area samples.?
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Twenty nine random samples were taken from the Rosario and Chincua colonies. The team then carried the butterflies down to a lower a elevation where it?s warmer--the Kust?s house in downtown of Angangueo. For several days they worked through the samples, separating the butterflies as alive, dead, or ?moribund.? (Moribund means the butterfly is not yet dead, but is flight impaired even when warm, and may die in the future.)
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Here are the data from the six samples from Chincua. Together they represent a combined area of 1.5 square meters. They counted 4,288 butterflies of which:
- 599 were still alive (13.9%).
- 327 were moribund (7.6%)
- and the rest, 3362 were dead (78%).
Thus the density of dead butterflies was 3,362 per 1.5
meter squared. This works out to be 2,241 dead per meter squared, or 22.41
million dead monarchs per hectare.
Population Estimates Before and After the Storm
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Post-storm population estimate assumes 80% morality at all colonies, based on preliminary data. |
- The population size BEFORE THE STORM
- The population size AFTER THE STORM
Fortunately, every winter Eligio Garcia, of Mexico?s Instituto
Nacional de Ecologia, measures the colonies, and had done so before the
storm. The size of the colony is estimated in two different ways: 1) by
measuring the area the colony occupies, and 2) by counting the number
of trees filled with butterflies. After the storm the Chincua and El Rosario
were revisited. (Other sites are still to be measured.) Here are the results:
| Sanctuary | BEFORE Storm | AFTER Storm | Percent Killed |
| Chincua | 2.96 hectares | 0.76 hectares | 74% |
| El Rosario | 2.69 hectares | Not Measured* | --- |
| El Rosario | 942 trees filled | 187 trees filled | 80% |
* In El Rosario it was not possible to
measure the area occupied by the butterflies after the storm, due to dispersal
of the colony into loosely aggregated clusters. For this reason, tree
counts were used.
The annual mid-winter population estimates, shown in the
graph above, were made by Eligio Garcia, of Mexico?s Instituto Nacional
de Ecolocia. We estimated the post-storm population size by assuming 20%
of the population survived. Much more information is needed to validate
this estimate; it?s based on observations at only two colonies, for example,
and the mortality rate might be much higher at the smaller, outlying colonies.
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Understandings About Science Inquiry
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Understand numbers, ways of representing numbers, relationships among
numbers, and number systems;
Compute fluently and make reasonable estimates.
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